A non-contact voltage detector is commonly used to detect the presence of alternating current (AC) voltage, typically high voltage, without requiring physical contact with the circuit. When a voltage is detected, the user is alerted by an indication, such as a light, buzzer, or vibrating motor. In contrast to two-pole testers, which maintain physical contact with a circuit at each pole, a non-contact detector cannot be driven by the measured voltage, and so a power supply such as a battery is needed to power the detector. However, if the power source is depleted, or if an internal circuit is broken or defective, then the indicator will fail to turn on. In this situation, the non-contact detector may provide a false negative as to the presence of a voltage on the circuit being tested, potentially subjecting a user to serious injury.